1. Field of the Invention
The invention relates to a backrest device for a chair, more particularly to a backrest device having a backrest member that can be maintained at an original height position after long-term use.
2. Description of the Related Art
Referring to FIG. 1, a conventional backrest device for a chair comprises a pair of spaced-apart backrest mounting pipe members 3, and a backrest unit 4.
Each of the pipe members 3 has a main surrounding wall 31 and a groove-defining wall 32. The main surrounding wall 31 defines a surrounding space 311 and is formed with an opening 323 extending longitudinally from an upper end to a lower end of the main surrounding wall 31. The groove-defining wall 32 is disposed in the surrounding space 311, has opposite wall ends connected respectively to opposite ends of the main surrounding wall 31 so as to define the opening 323 therebetween, and defines and engaging groove 321 therein. The engaging groove 321 of each of the pipe members 3 has a top open end 322 at the upper end 312 of the main surrounding wall 31 of a corresponding one of the pipe members 3.
The backrest unit 4 includes a backrest member 41 having opposite lateral portions 412, each of which forms a rod-receiving loop 411, and a pair of positioning rods 42 sleeved respectively in the rod-receiving loops 411.
When assembling the conventional backrest device, each of the lateral portions 412 of the backrest member 41 and the respective one of the positioning rods 42 are inserted downwardly into the engaging groove 321 of the respective one of the pipe members 3 through the top open end 322 of the engaging groove 321 of the respective pipe member 3 to thereby be retained within the respective engaging groove 321. As such, the backrest member 41 is stretched, and the lateral portions 412 and the corresponding positioning rods 42 are maintained at an original height position by virtue of the elasticity of the backrest member 41.
However, since the backrest member 41 may become loose after long-term use, the backrest member 41 has a tendency to fall toward lower ends of the main surrounding walls 31, thus resulting in inconvenience during use.
Furthermore, dust may easily accumulate in the surrounding spaces 311 of the pipe members 3 through the top open ends 322 of the pipe members 3.